In a known OCB mode, phase difference between long axes of liquid crystal molecules at surfaces of the upper and lower substrates is 180°. The liquid crystal molecules are in a splay state when no external voltage is supplied. When the external voltage is supplied, the liquid crystal molecules becomes in a bend state, in this case, the liquid crystal molecules closest to the upper and lower glass substrates are oriented substantially parallel to the substrates, while the liquid crystal molecules in the middle portion of a gap between the upper and lower glass substrates are perpendicular to the upper and lower substrates all the time. The other liquid crystal molecules are bent, but not twisted, in a plane. As the liquid crystal molecules in upper and lower portions are symmetrical to each other in bend state, influence on the optical performance caused by the oblique liquid crystal molecules is overcome, the liquid crystal molecules between the upper and lower substrates compensate for each other, thereby achieving a symmetrical wide viewing angle.
As illustrated in FIG. 1, as liquid crystal molecules 31 in OCB mode are rotated in only one plane, liquid crystal directors in the upper and lower portions are deflected in the same direction under the action of an electric field (E-field). By this means, backflow effect when adjusting the orientations of the liquid crystal molecules is avoided, thereby significantly improving the response speed of the liquid crystals.
However, nucleation is required when the OCB liquid crystal molecules switch from the splay state “b” to the bend state “a”. When a voltage is applied, some liquid crystal molecules first switch to the bend state and become transition nucleuses in the liquid crystal cell, and other liquid crystal molecules will gradually switch to the bend state by following the transition nucleuses. The bend plane extends bit by bit until all the liquid crystal molecules switch to the bend state, which takes a long time. In the case that the transition nucleuses are not formed, it is possible that the liquid crystal molecules will not switch to the bend state. Moreover, if the external voltage is smaller than a critical voltage, the liquid crystal molecules will still not stable after switching to the bend state and the liquid crystal molecules will switch back to the splay state instantly.
It is seen from the above that a large voltage (about 20V) or a long time is required to finish the transition from the splay state to the bend state.
There is currently some research on this issue. As an example, nucleation agents or symmetrical chiral molecules are mixed in the liquid crystal molecules. The molecules are in twist state when no voltage is applied. It takes a smaller amount of energy to switch from twist state to the bend state than from the splay state to the bend state, making the transition easier. Some research adds macromolecule polymer to the liquid crystal molecules; the macromolecule polymer is photosensitive and can polymerize the molecules when being irradiated. The liquid molecules in irradiated portion will be arranged in bend state and becomes the bend nucleuses.
However, the above researches require new liquid crystal materials developed and suitable photosensitive polymer found, making the development cost high.